1. Field of the Invention.
This invention relates to a method and apparatus for tempering glassware to produce residual compressive stresses on the glassware surfaces in order to increase the resistance of the glassware to mechanical and thermal stresses to which it is subjected during use.
2. Description of the Prior Art.
Glassware made of soda-lime-silica or other glass compositions may be tempered to produce residual compressive stresses on the glassware surfaces to increase the resistance of the glassware to mechanical and thermal stresses to which it may be subjected during use. The beneficial residual compressive stresses result from the establishment of significant temperature differentials through the thickness of the sections of the glassware after the glassware has been heated to a uniform temperature for a sufficiently long period of time. The glassware must be uniformly heated above the strain point of the particular glass composition used to form the item. The magnitudes of the temperature differentials established during the tempering process determine the magnitudes of the residual compressive stresses which increase the performance of the glassware.
The temperature differentials utilized to produce the tempering are typically established by directing high velocity streams of cooler air onto the uniformly heated glassware surfaces. An example of this method of tempering may be found in U.S. Pat. No. 3,155,481. U.S. Pat. Nos. 2,390,910 and 3,608,766 are also directed to tempering glassware, as are United Kingdom Pat. No. 810,174, French Pat. No. 833,559, and Italian Pat. No. 359,686.
While methods and apparatus for tempering glassware are known, none of the methods or apparatus provide an efficient procedure for directing cooling gas against a rotating glass article so that the surface of the glass article is efficiently subjected to residual compressive stresses that strengthen the glassware. Therefore, there is a need for a method and apparatus which provides for the efficient cooling of the surface of heated glassware in order to temper it effectively.